This invention relates to the separation of oil from water in wastewater containing an emulsified oil. More particularly, the invention relates to a method and an apparatus for performing oil-water separation by diaphragm electrolysis of water-base cleaning solutions, liquid waste water-soluble cutting oils and coolants that contain surfactants and oils at the same time.
As a result of the recent revision of the London Treaty on the ocean disposal of wastes, it is no longer possible to dump liquid waste water-soluble cutting oils and coolants into the sea which has been the method of disposal so far. Hence, it is urgently required to develop a technology that provides for economical land disposal of these kinds of wastewater.
Chlorine-base organic solvents such as Flon and trichloroethane which have hitherto been extensively used as industrial detergents are now recognized as ozone layer depleting substances and it was agreed internationally to prohibit the production of these substances by the end of 1995. Under these circumstances, the development of alternative detergents to Flon and trichloroethane is a peremptory need and they are being replaced by water-base detergents containing surfactants and alkalies as main components, quasi-water-base detergents comprising a mixture of water with organic solvents such as alcohols and glycol ethers, and non-water-base detergents typified by hydrocarbon-base solvents.
However, these substitute detergents have their own problems. To begin with, non-water-base detergents suffer from the disadvantage of high running costs since the cleaning operation is performed with these detergents alone. In addition, most of these non-water base detergents are inflammable, so that the cleaning apparatus must be designed to be explosion-proof (which increases its price) or cannot be of large size.
The water-base and quasi-water-base detergents, particularly, the water-base detergents, feature low running costs since they are diluted with large volumes of water before use. In addition, they are not potentially a hazard, so a large cleaning apparatus can be constructed easily at a fairly low cost. On the other hand, the use of large volumes of water requires that a water treatment unit capable of oil-water separation of cleaning solutions and ecologically acceptable discharge of rinse water should be installed as an essential component of the overall cleaning system. Consider, for example, the case of cleaning workpieces with water-base detergents. As the cleaning operation proceeds, contaminants such as oils from the workpiece build up gradually in the cleaning solution to reduce its detergency. Naturally, in order to extend the service life of the cleaning solution while maintaining its detergency, contaminants such as oils must be constantly removed from the cleaning solution.
Conventional methods of performing oil-water separation on water-base detergents include: an emulsion breaking and floating separation method which employs a chemical such as an emulsion breaker; an electrostatic separation method; a coalescer method which accelerates the coalescing and coarsening of oil particles; and a membrane separation method which employs an ultrafiltration membrane or a microfiltration membrane.
However, these conventional techniques have their own problems. In the emulsion breaking and floating separation method which employs an emulsion breaker, the cleaning solution which has been subjected to the oil-water separation treatment has no detergency and is not suitable for subsequent use. In the electrostatic separation method and the coalescer method, the intended effect of oil-water separation is not attained if the oil content of the cleaning solution is present as fine emulsion particles. The use of an ultrafiltration membrane or a microfiltration membrane has the disadvantage of removing not only the oil but also the detergent component and, in addition, an expensive apparatus has to be employed.
Water-base detergents are typically composed of surfactants as a main component which is responsible for detergency, as well as rust inhibitors, antifoaming agents, and organic or inorganic builders such as alkali components. The surfactants which are responsible for detergency may be nonionic or anionic but from a detergency viewpoint, nonionic surfactants having cloud points in the range from 30.degree. to 60.degree. C. are often used. Prior to use, the water-base detergents are diluted with water to a specified concentration, thereby formulating aqueous cleaning solutions.
At temperatures below their cloud points, nonionic surfactants dissolve in water and exhibit surface activity; however, at temperatures above their cloud points, the hydrophilic groups are dehydrated and the molecules associate with themselves to cause the loss of surface activity. At even higher temperatures, the nonionic surfactants precipitate either as a floc or in a liquid form. Conversely, if the temperature drops below the cloud point, the hydrophilic groups in the surfactant which have been insoluble are hydrated to become water soluble again, thereby restoring the surface activity.
Therefore, if an aqueous cleaning solution containing a nonionic surfactant as a main component is heated to a temperature higher than the cloud point, the surfactant will lose its surface activity and the oil content will float to separate from the water. However, if the cleaning solution has an oil contaminant, the surfactant will not precipitate on account of the interaction with the oil; on the contrary, it floats together with the oil content, making it impossible to discharge only the oil content from the system, which is the inherent object of the oil-water separation treatment.
There has been previously proposed a floating separation technology that could accomplish efficient oil-water separation of an aqueous cleaning solution even in the case where the latter was contaminated with an oil.
According to such technology, both a nonionic surfactant having a cloud point of 40.degree.-70.degree. C. and a nonionic surfactant having a cloud point of 20.degree.-40.degree. C. were incorporated in a water-base detergent and the contaminant such as oil in the cleaning solution could be separated by merely heating it to a temperature above the cloud point of the detergent. The technology also included a method for oil-water separation of the aqueous cleaning solution that has been used in the cleaning operation.
The technology works very effectively for cleaning solutions that contain water-insoluble oils and those which contain water-insoluble and non-emulsifiable oils; however, it has not been applicable to cleaning solutions that contain water-soluble oils or those which contain emulsifiable water-insoluble oils that incorporate anionic surfactants.
Oils used in the machining of metal parts consist of water-soluble oils typified by water-soluble cutting oils and coolants, and water-insoluble oils typified by press working oils and rolling mill oils. The water-insoluble oils are classified as an emulsifiable type and a non-emulsifiable type. The water-soluble oils typified by water-soluble cutting oils and coolants have high contents of anionic surface active substances such as sodium alkylsulfonates and some of the water-insoluble oils contain large amounts of calcium sulfonate and other anionic surface active substances as rust inhibitors. The water-base detergent previously developed and the method of oil-water separation based on the heating of such water-base detergent have been inapplicable to those oils which contain large amounts of anionic surface active substances.